ES2410567T3 - Linear actuator - Google Patents

Abstract

Linear actuator (1) that includes: - an external tubular body (2) that extends along an axis and which, internally, has at least one helical raceway (20); - an internal tubular body (3) movable in translation inside the external tubular body (2), coaxial to the latter (2), and internally receiving a rotating motor (40) of a coaxial shaft (41) to tubular bodies (2; 3); - mounted on this axis (41) at least one nut (42), mobile in rotation and translation inside the tubular-outer body (2), and which has, on the one hand, at least one helical raceway (420) that it extends around this nut (42) and provided with two ends and, on the other hand, a recirculation zone (421) that connects both ends of such a raceway (420); - balls (44) arranged between a raceway (420) of at least one nut (42) and a raceway (20) of the external tubular body (2) as well as in the recirculation zone (421) of such nut (42), characterized in that the elapser (1) comprises: - at least one support means (5), on the one hand, formed by a generally cylindrical skid (50) fixedly attached to the shaft (41) and having at least a helical thread (51) extending around this skate (50) and, on the other hand, designed to rest on a raceway (20) of the external tubular body (2) along at least one contact line; - at least one control means (6) for this support means (5), this control means (6), on the one hand, means (43) for the fixed mounting in rotation and mobile in translation of the o the nuts (42) on the shaft (41) and, on the other hand, axially and both sides of such a nut (42), at least one means (60; 60 ') a prestressed on such nut (42), this means being control (6) designed, on the one hand, to maintain such support means (5) in an inactive support position in which a play (53) remains between this support means (5) and the raceway (20) and , on the other hand, to make such support means (5) adopt an active support position on the raceway (20), under the effect of an axial tension (F) greater than a threshold tension (Fs) exerted on the actuator (1).

Description

Linear actuator.

The present invention relates to a linear actuator that comprises a ball screw that allows a rotation movement to be converted into a translation movement.

This invention relates to the field of manufacturing linear mechanical actuators, in particular those designed to ensure a drive under the impulse of an electric motor (electromechanical actuator).

Such a type of electromechanical actuator is already known that eliminates the inconveniences of hydraulic or pneumatic actuators, usually formed by jacks that require an external fluid source, difficult to control and that lack precision.

Such an electromechanical actuator generally comprises a screw and ball nut system as well as an electric motor designed to drive said nut in rotation which, by rotation, drives the screw in translation.

In particular, from document EP-1,523,630 an electromechanical actuator is known which comprises an external tubular body provided, internally, with at least one helical raceway, intended to ensure the guidance of balls that this actuator entails, and which confers on This external tubular body the structure of a screw.

This actuator also has an internal tubular body, movable in translation inside the external tubular body, and which, internally, comprises an electric motor for the rotation drive of a coaxial axis to the two tubular bodies.

The actuator also includes a nut, movable in translation and in rotation inside the tubular body, and mounted fixed in rotation on said axis for its actuation in rotation by the latter, under the impulse of the motor. This nut has, on the one hand, a helical raceway that extends around this nut and provided with two ends and, on the other hand, a recirculation zone that connects the two ends of this raceway. In fact, such a nut consists of several aligned elements, cylindrical in shape, and each having at least one chamfer. The immediately juxtaposed two-element chamfers define a raceway for the actuator balls.

This advantageous design of such an actuator allows to benefit from a circulation diameter of the center of the balls greater than that of the conventional actuators as well as using balls of diameter also greater. This helps to significantly reduce the Hertz pressure applied in a timely manner by these balls on the raceway for the same driving effort as that exerted in the case of a conventional actuator. It follows that the external tubular body is subjected, in normal operation of the actuator, to reduced pressures that advantageously allow such a body to be made of plastic material, in particular by molding.

However, such an actuator has drawbacks when it is subjected to static pressures that are more important than the dynamic pressures exerted by the actuator in normal drive operation and which result in particular from an impact suffered by this actuator. Indeed, under the effect of such static pressure the balls tend to lean heavily against the raceway and cause degradation thereof.

A linear actuator described in US-2,756,609 is also known, which constitutes the closest state of the art, and which comprises, on the one hand, an external tubular body that extends along an axis and, on the other hand, a internal tubular body, mobile in translation inside the external tubular body, and coaxial to the latter. This linear actuator also includes, on a coaxial axis to the tubular bodies, at least one nut, mobile in rotation and in translation inside the external tubular body and which, on the other hand, has at least one helical raceway that it extends around this nut and provided with two ends and, on the other hand, a recirculation zone that connects both ends of such a raceway. This linear actuator also includes balls arranged between a raceway of at least one nut and the external tubular body as well as in the recirculation zone of such nut.

The present invention aims to be able to eliminate the inconveniences of the actuators of the prior art.

To this end, the invention relates to a linear actuator that includes:

-a external tubular body that extends along an axis and which has, at least internally, a track of helical rolling; -a internal tubular body, mobile in translation inside the external tubular body, coaxial to the latter, and that internally receives a rotating drive motor from a coaxial axis to the tubular bodies;

-mounted on this axis, at least one nut, mobile in rotation and in translation inside the tubular body external, and which has, on the one hand, at least one helical raceway that extends around this nut and provided with two ends and, on the other hand, a recirculation zone that connects the two ends of such raceway; -balls arranged between a raceway of at least one nut and a raceway of the tubular body external as well as in the recirculation zone of such nut. According to the invention, this actuator includes: -at least one support means, on the one hand, consisting of a generally cylindrical skate, fixedly attached to the shaft and which has at least one helical thread that extends around this skate and, on the other hand, designed of so that it rests on a raceway of the external tubular body along at least one contact line; -at least a means of control of this means of support, with this means of control, on the one hand, a means for fixed mounting in rotation and mobile in translation of the axle nut (s) and, on the other hand, axially and both sides of such a nut, at least one means for exerting a prestress on such a nut, this means being control designed, on the one hand, to maintain such support means in an inactive support position in which a game remains between this support means and the raceway and, on the other hand, to make such a means of support take an active position of support on the raceway, under the effect of such axial pressure, greater than one threshold pressure, exerted on the actuator.

An additional feature refers to the means for exercising a claim consisting of an elastic element crushed to a dimension that corresponds to the threshold pressure.

Finally, another feature is that the or the nuts of this actuator have at least two cams:

-in general cylindrical shape; - axially aligned; -which each have at least one chamfer in an adapted way, made so that it defines a path of helical rolling around this nut and machining performed so that it defines an area of ball recirculation.

The actuator according to the present invention comprises a means designed to rest on a raceway of the external tubular body, according to at least one contact line. Such support allows advantageously to distribute the pressures exerted on such a raceway along a contact line and no longer limit the application of these pressures to specific areas that correspond to the point contact of a ball with this raceway.

Another advantage is that the actuator has a control means that ensures the maintenance of this support means in an inactive position in which this support means does not rest against the raceway and this does not prevent the drive when this actuator is at rest as well as in the operating frame normal actuator, but also when an axial pressure lower than a pressure is exerted on this actuator threshold.

On the contrary, this control means is designed to make the support means adopt an active position of bearing against the raceway, only when an axial pressure greater than a threshold value is exerted About this actuator. Such pressure corresponds, for example, to a static pressure exerted on the actuator and that results particularly from an impact suffered by this actuator, by the mobile element driven by this actuator or by the fixed element associated with this actuator.

Another advantage is that this means of support and this means of control are formed by elements that they have a simple design that allows easy intervention, particularly for placement, control or substitution.

Other objects and other advantages of the present invention will appear during the description that follows and refers to embodiments that are given only by way of indicative and non-limiting examples.

Understanding this description will be provided with reference to the attached drawings and in which:

FIG. 1 is a general and sectional view of an actuator according to the present invention; Figure 2 is a schematic and detailed view of the actuator illustrated in Figure 1 and depicting the nut, the support means as well as a first embodiment of the medium, in an inactive support position of the means of support for ; -Figure 3 is a view similar to the previous one and corresponds to the control medium and the support medium it adopts an active support position of the support medium; -Figure 4 is a schematic and detailed view of an actuator and representing the nut, the support means as well as a second embodiment of the control means, in the inactive support position of the support means; -Figure 5 is a schematic and perspective view of a support means according to the invention.

The present invention is related to the field of manufacture of mechanical linear actuators that involve a ball screw that makes it possible to convert a rotation movement into a translation movement.

Such an actuator 1 comprises an external tubular body 2 which extends along an axis and has, internally, at least one helical raceway 20 defined at the level of the internal wall 21 of this external tubular body 2.

In fact, such a raceway 20 can be made by deformation of this inner wall 21 or by molding during the manufacturing process of the tubular body 2. Another embodiment is that this raceway 20 can be defined by at least one wire (in particular high strength steel) formed in a helix and positioned inside the external tubular body 2, against the inner wall 21 of the latter 2.

This actuator 1 also comprises an internal tubular body 3 which extends, here also, along an axis that preferably corresponds to the axis of the external tubular body 2, so that these tubular bodies (2; 3) are coaxial.

In fact, this internal tubular body 3 extends, at least in part, inside the external tubular body 2. The actuator 1 is designed so that these tubular bodies (2; 3) are fixed in rotation, but mobile in translation , so that they allow a sliding of the internal tubular body 3 with respect to the external tubular body 2 and internally to the latter 2, according to the longitudinal direction of these tubular bodies (2; 3).

The actuator 1 also includes a means 4 designed to slide these tubular bodies (2; 3) one 2 with respect to the other 3.

Such drive means 4 comprises, on the one hand, an electric motor 40 whose stator is fixedly mounted inside the internal tubular body 3.

On the other hand, this drive means 4 comprises a drive shaft 41, connected in rotation to the rotor of the electric motor 40 (either directly, or through a speed reducer), for its drive in rotation, and extends along an axis that preferably corresponds to the axis of the tubular bodies (2; 3).

This drive shaft 41 extends at least partly inside the internal tubular body 3 in which 3 is guided by at least one bearing 30, in particular a ball bearing, which internally comprises this internal tubular body 3 and whose ring The inner ring is fixedly fixed in rotation on the shaft 41 while the outer ring is mounted fixedly in rotation inside the inner tubular body 3, in particular on the inner wall 31 of the latter 3.

As visible in the attached figures, this drive shaft 41 also extends outside of this inner tubular body 3 as well as inside the outer tubular body 2 of the actuator 1.

The drive means 4 also includes at least one nut 42 which has, on the one hand, at least one helical raceway 420 which extends around this nut 42 and provided with two ends and, on the other hand, a recirculation zone 421 connecting the two ends of such a raceway 420.

In addition, this drive means 4 includes a means 43 for the fixed mounting in rotation of such a nut 42 on the drive shaft 41. This mounting means 43 then allows such a nut 42 to be mounted in a mobile manner in rotation as well as in translation. inside the external tubular body 2.

Finally, this actuation means 4 comprises a plurality of balls 44 arranged between a helical raceway 420 of at least one nut 42 and a raceway 20 of the external tubular body 2 as well as in the recirculation zone 421 of such nut 42 .

This drive means 4 is then designed so that the electric motor 40 drives the drive shaft 41 in rotation, which, in turn, drives at least one nut 42 in rotation. Such a nut 42, under the effect of its rotation, drives the balls 44 that move inside the helical raceway 420 of this nut 42 as well as inside the raceway 20 of the external tubular body 2, thereby causing the internal tubular body 3 to slide relative to this external tubular body 2.

According to the invention, this actuator 1 comprises at least one means 5 associated with the shaft 41 and designed to rest on a raceway 20 of the external tubular body 2 according to at least one contact line.

For this purpose and as visible in the attached figures, this support means 5 is in the form of a skid 50 in a generally cylindrical shape that extends along an axis that preferably corresponds to the axis of the tubular bodies (2; 3).

Such a skate 50 is fixedly connected (in rotation and in translation) to the drive shaft 41, in particular to the free end of this shaft 41, opposite to that which meshes with the electric motor 40.

Another feature of this skate 50 is that it has at least one helical thread 51 extending around this skate 50, in particular at the level of the skate 50. This helical thread 51 has a passage substantially equal to the passage of the helical raceway 20 of the external tubular body 2.

As visible in the attached figures, this helical thread 51 has a profile that adopts approximately a general semicircle shape whose radius is substantially equal to that of the balls 44.

More precisely, this helical thread 51 has in fact a profile that takes the form of two circle arcs 52 whose centers are preferably offset and whose radii correspond substantially to that of the balls

44.

For this purpose, it will be noted that the centers of the circles of these two circle arcs 52 are axially offset, in particular along an axis parallel to the axis of the external tubular body 2 and / or passing through the center of a rolling ball 44 on the raceway 20 of this external tubular body 2.

In fact, the offset between the centers of the circles of the two arcs of circles 52 corresponds substantially to a lateral clearance 53 defined between the thread 51 of the skate 50 and the raceway 20 of the external tubular body 2.

In particular, the portions of the thread 51 of the skate 50 delimited by these two circle arcs 52 are able to rest against the raceway 20 of the external tubular body 2 (in particular against the sides of this track 20), according to at least A contact line.

For this purpose, it should be noted that this raceway 20 has a profile that takes the form of a semicircle whose radius corresponds substantially to that of the balls 44 that roll on this track 20. The profile of this raceway 20 then also corresponds to the profile of the two circle arcs 52 of the helical thread 51 of the skate 50, so that this skate 50 is able to rest on this raceway 20 according to a contact line that has a certain width and that in fact appears in particular to a contact band whose width substantially corresponds to the length of such a circle arc 52.

Another characteristic of the helical thread 51 of the skate 50 consists in that the apex 54 of this thread 51 has at least a truncated portion so as to avoid the contact of this apex 54 with the raceway 20 (in particular with the bottom of the raceway 20) of the outer tubular body 2.

As regards the support of the support means 5 on the raceway 20 of the external tubular body 2, this is in fact the result of an external pressure exerted on the actuator 1 (in particular on the external tubular body 2 and / or on the tubular body internal 3 of this actuator 1). Such external pressure corresponds to an accidental pressure (other than a pressure exerted on this actuator 1 within the framework of its normal operation of operation) which consists in particular of a static pressure, which results in particular from an impact suffered by this actuator 1.

For this purpose, it will be noted that such external pressure has an axial component F that extends along the extension axis of the tubular bodies (2; 3) and of the drive shaft 41.

According to the invention, the actuator 1 also includes at least one means for controlling 6 of the aforementioned support means 5.

This control means 6 is designed to make such support means 5 adopt an active support position on the raceway 20 (according to at least one contact line, as described above), under the effect of a pressure axial F, greater than a threshold pressure Fs, exerted on actuator 1 (figure 3).

This control means 6 is also designed to maintain such support means 5 in an inactive support position in which a play 53 (on each side of the thread 51) remains between this support means 5 and the raceway 20, in the absence of depression or for an axial pressure F lower than said threshold pressure Fs (figures 2 and 4).

As mentioned above, the drive means 4 includes a mounting means 43 of at least one nut 42 on the drive shaft 41.

In fact and according to a particular feature of the invention, the control means 6 of the support means 5 is formed, at least in part, by this mounting means 43 which, according to the invention, is designed to ensure a fixed mounting in rotation and mobile in translation, of the nut (s) 42 on the axis 41.

For this purpose, it will be noted that, for such assembly, this mounting means 43 includes, by way of example, at least one pin designed to cooperate with a groove provided at least at the level of a nut 42, even at the level of axis 41 as well as at the level of the support means 5 (figure 5).

Another feature is that such mounting means 43 of a nut 42 comprises side stops (430, 430 ’) between which such nut 42 is mounted in translation.

In fact, and as visible in the attached figures, at least one such lateral stop 430 can be formed by a shoulder associated with the drive shaft 41. Such a shoulder can then be defined at the level of the shaft body 41 or also defined at level of a tightening nut 70 (described in more detail below) axially mounted on this shaft 41.

In addition, one of these lateral stops 430 ’can be formed by the aforementioned support means 5.

In the attached figures there is shown a preferred embodiment of the invention corresponding to an actuator 1 which comprises a single nut 42 mounted on the axis 41 in a mobile way in translation between two lateral stops (430, 430 ’). In this embodiment, one 430 'of these lateral stops is then formed by the support means 5, while the other 430 is formed by a nut 7 associated with the axis 41.

A further feature of the invention is that the control means 6 comprises, on both sides of at least one nut 42, at least one means (60; 60 ’) for exerting a prestress on such nut 42.

In fact, such a prestressing means (60; 60 ’) is axially positioned, in particular along the drive axis of the external 2 and internal tubular bodies 3, with respect to the drive shaft 41.

According to a preferred embodiment of the invention illustrated in the attached figures, such means (60; 60 ') for exercising a prestress is interposed between a lateral stop (430; 430') and a nut 42 of the actuator 1. In particular, on such a lateral stop (430; 430 ') and on such nut 42 such a prestressing means (60; 60') is supported, to exercise such prestressing on such nut 42.

In fact, a preferred embodiment consists in that a means 60 for exerting a prestress is formed by an elastic element crushed on one side corresponding to the threshold tension Fs.

Such elastic element may be formed by a spring (in particular a helical spring), even (and preferably) by a Belleville type washer.

Thus, when the tension exerted on the actuator 1 is lower than the threshold voltage Fs, the shaft 41 and the nut 42 adopt an inactive support position (Figures 2 and 4).

However, when the tension exerted on the actuator 1 exceeds the threshold voltage Fs, the shaft 41 will slightly offset with respect to the nut 42 in support on the balls 44 until the excess tension (with respect to the threshold voltage Fs) compresses the prestressing means 60 of the value of the game 53 between the support means 5 and the raceway 20.

The support means 5 is then supported on the raceway 20 that receives the stress (Figure 3). This support is made according to at least one contact line representing a contact surface considerably larger than that of the contact points of the balls 44 with the raceway 20.

In addition, these balls 44 suffer only a limited effort due to the threshold tension Fs to which the necessary effort delta is added to ensure the displacement of the shaft 41 for the support of the support means 5 in the raceway 20. The pressure of Hertz resulting from the contact points between the balls 44 and the raceway 20 is then limited, especially in comparison to the pressure exerted by the support means 5 on this raceway 20.

Most of the voltage received by the actuator 1 is then exerted at least on the contact line between the support means 5 and the raceway 20, which allows a better distribution of this tension on the actuator 1 (with respect to the same voltage exerted only through contact points, as is the case for the state of the art actuators) and, consequently, limit (even avoid) the degradations that can be caused to this actuator 1 by such tension .

A further feature is that the actuator according to the invention comprises a means 7 for adjusting the prestressing exerted by a prestressing means (60; 60 ') on at least one nut 42. Such adjustment means 7 comprises an associated tightening nut 70 with the drive shaft 41 (in particular axially mounted on the latter 41), which defines a shoulder that constitutes a said lateral stop 430, and which rests against such prestressing means 60.

Another feature of the invention is that the nut (s) 42 that the drive means 4 has features that are described in more detail in EP-1,523,630.

In fact, such a nut 42 has at least two cams (422, 422 ’) that adopt a general cylindrical shape and extend, each one, along an axis that preferably corresponds to the axis of the tubular bodies (2; 3).

As visible in the attached figures, these cams (422, 422 ’) are axially aligned along that axis and are positioned one 422 opposite the other 422’.

These cams (422; 422 ’) each have at least one chamfer adapted in an adapted manner so as to define a helical raceway 420 around the nut 42 formed by these cams (420; 420’).

The latter (422; 422 ’) also each have a machining performed so as to define a recirculation zone 421 of the balls 44.

A particular embodiment consists in the fact that the nut 42 of an actuator 1 has, in fact, two cams (422; 422 ’) having a symmetrical structure, even (and preferably) identical.

A further feature of the invention is that the actuator 1 (in particular its mounting means 43) has a plurality of stops (431; 431 ') each associated with a cam (422; 422') of the nut 42 In particular, such a cam (422; 422 ') is held in support against such a stop (431; 431') by means (60; 60 '), to exercise a prestress.

Thus, in Figures 1 to 3 a first embodiment of the invention has been represented (which constitutes, in particular a symmetrical solution) in which such a stop (431; 431 ') is formed by a shoulder defined at the body level of axis 41 and against which a cam (422; 422 ') is held in support.

In fact, such maintenance is ensured by an axial tension exerted on the prestressing means (60; 60 ’) associated with this cam (422; 422’), less than or equal to the threshold voltage Fs. In particular and as visible in Figures 1 and 2, such maintenance is ensured in the absence of tension exerted on such a prestressing means (60; 60 ’).

As visible in the attached figures 1 to 3, the stops (431; 431 ') associated with the cams (422; 422') of the same nut 42 are interposed between two cams (422; 422 ') of this nut 42 , they are even formed by a single element (in particular formed by a radial rib) at the level of which two projections are defined that constitute such stops (431; 431 ').

In Fig. 4 a second embodiment (which in particular constitutes a differential solution) has been shown in which a stop (431; 431 ') associated with a cam (422; 422') of a nut 42 is in fact formed by another cam (422 '; 422) that this nut 42 behaves and against which (422'; 422) this cam (422; 422 ') is held in support.

Here too, such maintenance is ensured by an axial tension exerted on the prestressing means (60; 60 ’) associated with this cam (422; 422’) less than or equal to the threshold voltage Fs. In particular and as visible in Figure 4, such maintenance is ensured in the absence of tension exerted on such a prestressing means (60; 60 ’).

In fact, each cam (422; 422 ’) of the same nut 42 can rest on the other cam (422 ′; 422) of this nut 42, directly or indirectly.

For this purpose and as visible in FIG. 4, the actuator 1 according to the invention may, interposed between two cams (422; 422 ') of the same nut 42, an intermediate element 8 on which each of them rests these two cams (422; 422 '), in particular under the impulse of the means (60; 60') to exert a prestress on this nut 42, for the indirect support of such a cam (422; 422 ') on the other (422 '; 422).

A further feature of the invention is that the actuator 1 also includes a means 8 for adjusting a prestress exerted by these cams (422; 422 ') on the balls 44.

In fact, such adjustment means 8 is in particular designed to exercise a base prestress on these balls 42 by means of these cams (422; 422 ’).

A preferred embodiment is that this adjustment means 8 is interposed between two cams (422; 422 ') of the same nut and is in particular formed by the aforementioned intermediate element 8.

According to a preferred embodiment, this adjustment means 8 can be formed by a washer inserted on the drive shaft 41.

In particular, this adjustment means 8 can advantageously be designed to always exercise the same prestressing on the balls 42, even in case of wear. To this end, this adjustment means 8 can be of the elastic type and then formed by an elastic washer.

Finally, within the framework of this second embodiment (differential solution) and as visible in Figure 4, the drive shaft 41 that receives the cams (422; 422 ') of at least one nut 42 is preferably of type on steps

An actuation shaft 41 of this type then advantageously allows an easy assembly (as well as disassembly) of the various elements that constitute the actuation means 4, the support means 5 and the control means 6, in the internal tubular body 3 .

Claims (11)

1. Linear actuator (1) that includes:

-

 an external tubular body (2) which extends along an axis and which has, inwardly, at least one helical raceway (20);

-

 an internal tubular body (3) movable in translation inside the external tubular body (2), coaxial to the latter (2), and which internally receives a rotating motor (40) of a coaxial shaft (41) a the tubular bodies (2; 3);

-

 mounted on this shaft (41) at least one nut (42), mobile in rotation and translation inside the external tubular body (2), and which has, on the one hand, at least one helical raceway (420) that it extends around this nut (42) and provided with two ends and, on the other hand, a recirculation zone (421) that connects both ends of such a raceway (420);

-

 balls (44) disposed between a raceway (420) of at least one nut (42) and a raceway (20) of the external tubular body (2) as well as in the recirculation zone (421) of such nut ( 42), characterized in that the actuator (1) includes:

-

 at least one support means (5), on the one hand, formed by a generally cylindrical skate (50) fixedly attached to the shaft (41) and having at least one helical thread (51) extending around this skate (50) and, on the other hand, designed to rest on a raceway (20) of the external tubular body (2) according to at least one contact line;

-

 at least one control means (6) for this support means (5), this control means (6), on the one hand, means (43) for the fixed mounting in rotation and mobile in translation of the or nuts (42) on the shaft (41) and, on the other hand, axially and both sides of such nut (42), at least one means (60; 60 ') a prestressed on such nut (42), this means being control (6) designed, on the one hand, to maintain such support means (5) in an inactive support position in which a play (53) remains between this support means (5) and the raceway (20) and, on the other hand, to make such support means (5) adopt an active support position on the raceway (20), under the effect of an axial tension (F) greater than an exerted threshold tension (Fs) on the actuator (1).

2.

 Linear actuator (1) according to claim 1, characterized in that the helical thread (51) has a passage substantially equal to that of the helical raceway (20) of the external tubular body (2).

3.

 Linear actuator (1) according to any of the preceding claims, characterized in that the helical thread (51) has a profile that adopts approximately a general semicircle shape (52) whose radius is substantially equal to that of the balls (44).

Four.

 Linear actuator (1) according to any of the preceding claims, characterized in that the helical thread (51) has a profile that takes the form of two circle arcs (52) whose centers are offset and whose radii correspond substantially to that of the balls (44).

5.

 Linear actuator (1) according to claim 4, characterized in that the offset between the centers of the circles of the two circle arcs (52) substantially corresponds to a lateral play (53) between the thread (51) of the skate (50) and the raceway (20) of the external tubular body (2).

6.

 Linear actuator (1) according to any of the preceding claims, characterized in that the apex (54) of the thread (51) has at least a truncated portion so as to avoid contact of this apex (54) with the raceway (20) of the external tubular body (2).

7.

 Linear actuator (1) according to any of the preceding claims, characterized in that the means (43) for mounting a nut (42) includes lateral stops (430; 430 ') between which such nut (42) is mounted in translation and that a means (60; 60 ') for exercising a prestress is interposed between such a lateral stop (430; 430') and the nut (42).

8.

 Linear actuator (1) according to any of the preceding claims, characterized in that the means (60; 60 ') for exerting a prestress is formed by an elastic element crushed on one side corresponding to the threshold tension (Fs).

9.

Linear actuator (1) according to any of the preceding claims, characterized in that the nut (s)

(42) have at least two cams (422; 422 ') in a general cylindrical shape, axially aligned and each having at least one chamfer made so as to define a helical raceway (420) around this nut ( 42) and a machining performed so as to define a recirculation zone (421) of the balls (44). 10. Linear actuator (1) according to claim 9, characterized in that the actuator (1) comprises a plurality of stops (431; 431 ') each associated with a cam (422; 422') of the nut (42 ) and that each cam (422; 422 ') is held in support against such a stop (431; 431') by means (60; 60 ') to exercise a prestress. 11. A linear actuator (1) according to claim 10, characterized in that the stop (431; 431 ') is formed by a shoulder defined at the level of the shaft body (41) and against which a cam (422; 422' ) is held in support, for an axial tension exerted on the prestressing means (60; 60 ') associated with this cam (422; 422') less than or equal to the threshold tension (Fs). 12. Linear actuator (1) according to claim 10, characterized in that the stop (431; 431 ') associated with a cam (422; 422') of a nut (42) is formed by another cam (422 '; 422 ) that this nut (42) behaves and against which (422 '; 422) this cam (422; 422') is held in support, for an axial tension exerted on the prestressing means (60; 60 ') associated with this cam (422; 422 ') less than or equal to the threshold voltage (Fs). 13. A linear actuator (1) according to claim 12, characterized in that the actuator (1) comprises, interposed between two cams (422; 422 ') of the same nut (42), a means (8) for adjusting the a prestressed by these cams (422; 422 ') on the balls (44). 14. Linear actuator (1) according to any of claims 12 or 13, characterized in that the shaft (41) received by the cams (422; 422 ') of at least one nut (42) is of the step type.